ing which chemical energy is released in the form of heat). Therefore, some
heat is transferred from the combustion chamber to the surroundings during
this process, which is 393,520 kJ/kmol CO 2 formed. (When one is dealing
with chemical reactions, it is more convenient to work with quantities per
unit mole than per unit time, even for steady-flow processes.)
The process described above involves no work interactions. Therefore,
from the steady-flow energy balance relation, the heat transfer during this
process must be equal to the difference between the enthalpy of the products
and the enthalpy of the reactants. That is,
(15–5)Since both the reactants and the products are at the same state, the enthalpy
change during this process is solely due to the changes in the chemical com-
position of the system. This enthalpy change is different for different reac-
tions, and it is very desirable to have a property to represent the changes in
chemical energy during a reaction. This property is the enthalpy of reac-
tionhR, which is defined asthe difference between the enthalpy of the prod-
ucts at a specified state and the enthalpy of the reactants at the same state
for a complete reaction.
For combustion processes, the enthalpy of reaction is usually referred to
as the enthalpy of combustionhC, which represents the amount of heat
released during a steady-flow combustion process when 1 kmol (or 1 kg)
of fuel is burned completely at a specified temperature and pressure
(Fig. 15–17). It is expressed as
(15–6)which is 393,520 kJ/kmol for carbon at the standard reference state. The
enthalpy of combustion of a particular fuel is different at different tempera-
tures and pressures.
The enthalpy of combustion is obviously a very useful property for ana-
lyzing the combustion processes of fuels. However, there are so many dif-
ferent fuels and fuel mixtures that it is not practical to list hCvalues for all
possible cases. Besides, the enthalpy of combustion is not of much use
when the combustion is incomplete. Therefore a more practical approach
would be to have a more fundamental property to represent the chemical
energy of an element or a compound at some reference state. This property
is the enthalpy of formationh
- f, which can be viewed as the enthalpy of a
substance at a specified state due to its chemical composition.
To establish a starting point, we assign the enthalpy of formation of all
stable elements (such as O 2 ,N 2 ,H 2 , and C) a value of zero at the standard
reference state of 25°C and 1 atm. That is,h
f0 for all stable elements.
(This is no different from assigning the internal energy of saturated liquid
water a value of zero at 0.01°C.) Perhaps we should clarify what we mean
by stable. The stable form of an element is simply the chemically stable
form of that element at 25°C and 1 atm. Nitrogen, for example, exists in
diatomic form (N 2 ) at 25°C and 1 atm. Therefore, the stable form of nitro-
gen at the standard reference state is diatomic nitrogen N 2 , not monatomic
nitrogen N. If an element exists in more than one stable form at 25°C and
1 atm, one of the forms should be specified as the stable form. For carbon,
for example, the stable form is assumed to be graphite, not diamond.
hRhCHprodHreactQHprodHreact393,520 kJ>kmolChapter 15 | 763Combustion
process1 kmol CO 2hC = Q = –393,520 kJ/kmol C1 kmol O 2 25 °C, 1 atm
25 °C, 1 atm1 kmol C
25 °C, 1 atmFIGURE 15–17
The enthalpy of combustion represents
the amount of energy released as a
fuel is burned during a steady-flow
process at a specified state.